Long Term Evolution-Unlicensed (LTE-U) refers to deployment of LTE in an unlicensed carrier for meeting increasing capacity requirements of a wireless communication system and improving a use efficiency of an unlicensed spectrum, and is a possible important evolution direction of LTE and future wireless communications. During design of the LTE-U, it is necessary to consider how to fairly and friendly contend, with inter-systems such as a Wireless Fidelity (WiFi) and a radar and LTE-U intra-systems, for an unlicensed spectrum to perform data transmission. And it is also necessary to avoid influencing and retaining LTE technical characteristics as far as possible. According to an expression of a 3rd Generation Partnership Project (3GPP) standard session, an LTE-U system is also referred to as an LTE Licensed Assisted Access (LAA) system.
A communication system using an unlicensed carrier avoids using an unlicensed carrier that is currently used by an existing station in unlicensed carriers, and otherwise, inter-system interference will be caused. Therefore, in some countries (e.g., Europe and Japan), unlicensed carriers are forcedly required to support a Listen before Talk (LBT) function. Before a certain unlicensed carrier is used, it is necessary to execute a Clear Channel Assessment (CCA) function. If it is discovered that a certain device is using the unlicensed carrier or it is detected that signal energy exceeds a CCA threshold, an access is delayed. And if it is discovered that a channel is clear or it is detected that the signal energy is lower than the CCA threshold, the unlicensed carrier is occupied.
Therefore, LBT or CCA will limit from transmitting an LAA reference signal before occupying an unlicensed carrier. According to stipulations of the EN 301 893 of an European Telecommunications Standards Institute (ETSI), a control frame (e.g., Acknowledgment/Negative Acknowledgement (ACK/NACK) signal) is transmitted and managed by utilizing a Short Control Signaling (SCS). And moreover, it is unnecessary to perceive that a current channel is clear. That is, it is unnecessary to execute the LBT/CCA before transmission. However, transmission of the SCS is strictly limited as follows: if the SCS is used, a maximum short message transmission duty ratio of an adaptive device does not exceed 5% within an observation period of 50 ms, i.e., 2.5 ms.
To use the unlicensed carrier, it is likewise necessary to solve problems of cell discovery, synchronization, Radio Resource Management (RRM) and the like, and a DS specified by a 3GPP Rel-12 is serving as a research reference.
Components of the DS defined by a Rel-12 at present are: a Primary/Secondary Synchronization Signal (PSS/SSS), a Cell-specific Reference Signal (CRS), and a Channel State Information-Reference Signal (CSI-RS), which are configurable. DS duration within each period under a Frequency Division Duplex (FDD) mode includes one to five successive subframes. DS duration within each period under a Time Division Duplex (TDD) mode includes two to five successive subframes.
In each component signal, a CRS is transmitted on all downlink subframes and a Downlink part of the special subframe (DwPTS) through an antenna port 0. A PSS is transmitted on a first subframe within each period of FDD or a second subframe within each period of TDD. An SSS is transmitted on a first subframe within each period, and a non-zero-power CSI-RS is transmitted on zero or more subframes within each period.
According to the related art, it is discovered as follows (actually, a Resource Element (RE) position or pattern of each component signal of the DS are consistent with an RE position or pattern of each independent signal before the Rel-12).
For FDD, an existing DS duration includes at least one subframe. In the at least one subframe, a CRS occupies a symbol 0, a symbol 4, a symbol 7 and a symbol 11, an SSS occupies a symbol 5, a PSS occupies a symbol 6, and a CSI-RS is configurable. If the CSI-RS is not configured, the DS crosses at least 12 symbols, and actually occupies at least six symbols.
For TDD, an existing DS duration includes at least two subframes. In the at least two subframes, a CRS occupies symbols 0, symbols 4, symbols 7 and symbols 11 in the first subframe and the second subframe, an SSS occupies a symbol 13 in the first subframe, a PSS occupies a symbol 2 in the second subframe, and a CSI-RS is configurable. If the CSI-RS is not configured, the DS crosses at least 26 symbols, and actually occupies at least 10 symbols.
Use of the unlicensed carrier is special. Limitations of necessity in execution of an LBT or CCA mechanism and maximum occupancy duration exist before the unlicensed carrier is used. Internal multi-symbol non-successive transmission of the DS involves a problem of repeated occupation (if symbol-based transmission) or over-long occupation time (the whole DS duration is reserved once and it is necessary to transmit an occupied or reversed signal). And it is required to re-design a pattern of a DS so as to reduce the number of times of occupation or to shorten occupancy duration.
The SCS has a strict time requirement for an SCS transmission manner without LBT or CCA in a European region. That is, a maximum short message transmission duty ratio of an adaptive device does not exceed 5% within an observation period of 50 ms, i.e., 2.5 ms. It is also required to shorten the occupancy duration of the DS, and moreover, internal multi-symbol non-successive transmission of the DS will cause multi-interference to a user who is contending for resources or has occupied the resources. Therefore, it is also necessary to re-design a pattern of the DS so as to shorten the occupancy duration, multi-interference to other devices is avoided, and some SCS resources are taken for transmitting other necessary short signaling.
To sum up, it is necessary to re-design an Unlicensed Carrier-Discovery Signal (UCDS) in the related art. On a premise of meeting and matching special requirements for use of an unlicensed carrier, occupation of DS resources is minimized, and a problem of multi-interference is avoided.